‘Pinning down’ how salty droplets dry

Tokyo, Japan – Researchers from Tokyo Metropolitan University have discovered a new way of controlling the drying patterns formed by re-crystallizing salt. They found that the coffee ring effect can be used to pin the edge of drying droplets, creating a range of different geometric patterns. The same principles may be applied to understand and improve the adhesion of printer ink to surfaces and the manufacture of film-based devices.

Anyone who has visited the beach this summer would have felt large chunks of salt form on their skin after a splash in the sea. These large, crystalline chunks are formed by re-crystallization as the sea water dries off and leaves the salt behind. The drying of salt solution is actually a very complex phenomenon involving the interplay of many variables, including concentration and density profiles, heat transfer, as well as a wide range of environmental factors such as temperature and humidity. Understanding and controlling the mechanisms behind re-crystallization is crucial to understanding drying-related industrial processes like the adhesion of printer ink, the manufacture of devices based on thin films, as well as phenomena like salt damage in brick and the dissolving of pharmaceuticals in the human body.

Drying droplets of solid-laden solution often leaves large, uneven chunks deposited at the edge. This is a fairly common sight; take the ring-like deposits left by spilt coffee. This so-called coffee ring effect is a result of different rates of evaporation on the top and at the edge of droplets, leading to a flow inside the droplet which drives an accumulation of solid particles at the edge. Ultimately, the droplet edge retracts, leaving a deposited ring. Though this effect is useful in concentrating the solute, it can be a nuisance when we are after a uniform coating.

Yet, a team from Tokyo Metropolitan University led by Associate Professor Rei Kurita showed that the same effect can be leveraged to achieve a very different effect. By artificially adding microscopic latex particles to droplets of salt solution, they found that the coffee ring effect took particles to the edge which subsequently pinned the droplet rim, keeping the droplet circular for the whole duration of the drying process as the salt re-crystallized. This led to the formation of beautiful, snowflake-like patterns, including dendritic, radial and concentric geometries, depending on temperature and humidity. The variety of behavior they uncovered is in stark contrast to the random re-crystallized chunks observed in drying droplets without these particulate additions. By systematically varying initial salt concentration and evaporation rate, they found that they could tune the morphology. High initial concentrations and slower vaporization led to radial patterns, while the opposite led to ridged, concentric patterns.

Further to the promise of new ways of artificially controlling drying in a wide range of industrial processes, the team hopes to build on their success to gain a deeper understanding of the mechanisms behind the dynamic processes underpinning this fascinating phenomenon.

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This work was supported by JSPS KAKENHI Grant-in-Aid (18J21231) for Young Scientists (17K14356) and for Scientific Research (B) (17H02945). The study has been published online in the journal Scientific Reports.